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Ammonia loss following nitrogen fertilization can degrade air quality and impact human health, whereas nitrous oxide (N2O) can contribute to global warming and climate change. Mitigation practices that target only one N‐loss pathway can lead to pollution swamping; hence, practices targeting both N‐losses are required. A 3‐year field study examined fertilizer N‐placement (broadcast urea, single‐slot injection of urea ammonium nitrate [UAN], double‐slot UAN injection) and N‐metabolization inhibitors (with/without urease and nitrification inhibitors) impacts on NH3 and N2O losses and corn yields. Ammonia volatilization was reduced (p < 0.05) by 26% with single‐slot UAN injection (10.6 kg N ha−1) and by 63% with double‐slot UAN injection (5.32 kg N ha−1) compared to broadcast urea (14.3 kg N ha−1). Dual urease and nitrification inhibitors reduced NH3 volatilization (0.84–3.86 kg N ha−1) by 57%–92% compared to no inhibitors (5.32–14.3 kg N ha−1). When no inhibitors were applied, N2O emissions from slot injection (6.43–7.62 kg N ha−1) were 2.6–3.1 times greater than from broadcast urea (2.43 kg N ha−1). Dual inhibitors reduced N2O emissions by 43% from 6.43 to 3.66 kg N ha−1 with double‐slot injection. Double‐slot UAN injection increased corn grain yields (9.73 t ha−1) by 12%–13% compared to single‐slot UAN injection (8.71 t ha−1) and broadcast urea (8.6 t ha−1). Double‐slot UAN injection effectively decreased NH3 losses and increased corn grain yields, but dual N inhibitors were required to also reduce N2O. Hence, combined productivity and environmental benefits were accrued only when fertilizer containing urease and nitrification inhibitors was combined with double‐slot injection. Core Ideas: Single‐slot and double‐slot UAN injection reduced NH3 loss by 26% and 63%, respectively, compared to broadcast urea.Dual inhibitors with broadcast urea or UAN injection reduced NH3 losses by 57%–92% compared to no inhibitors.Dual inhibitors reduced N2O emissions by 18% with single‐slot injection and by 43% with double‐slot injection.Double‐slot injection increased corn grain yields by 12%–13% compared to broadcast urea and single‐slot injection.Double‐slot injection with dual inhibitors increased yields and resulted in dramatic decreases in NH3 and N2O. [ABSTRACT FROM AUTHOR]

Conservation agriculture is often assumed to reduce soil N2O emissions. Yet, studies analyzing the specific effect of conservation agriculture practices on N2O emissions give contradictory results. Herein, we synthesized a comprehensive database on the three main conservation agriculture practices (cover crops, diversified crop rotations, and no-till and/or reduced tillage (NT/RT)) to elucidate the role of conservation practices on N2O emissions. Further, we used a random meta-forest approach to identify the most important predictors of the effects of these practices on soil N2O emissions. Averaged across all comparisons, NT/RT significantly decreased soil N2O emissions by 11% (95% CI: –19 to –1%) compared to conventional tillage. The reductions due to NT/RT were more commonly observed in humid climates and in soils with an initial carbon content < 20 g kg–1. The implementation of cover crops and diversified crop rotations led to variable effects on soil N2O emissions. Cover crops were more likely to reduce soil N2O emissions at neutral soil pH, and in soils with intermediate carbon (~20 g kg–1) and nitrogen (~3 g kg–1) contents. Diversified crop rotations tended to increase soil N2O emissions in temperate regions and neutral to alkaline soils. Our results provide a comprehensive predictive framework to understand the conditions in which the adoption of various conservation agriculture practices can contribute to climate change mitigation. Combining these results with a similar mechanistic understanding of conservation agriculture impacts on ecosystem services and crop production will pave the way for a wider adoption globally of these management practices. [ABSTRACT FROM AUTHOR]

Summer-seeding legume cover crops can reduce erosion, mitigate nitrate leaching losses during the non-growing season, and provide bio-accumulated nitrogen (N) to the following crop. Very little information is available, however, on the most effective methods for terminating summer-seeded cover crops, or on the amount of N they can provide to subsequent crops. We therefore evaluated the impacts of selected legume cover crops and cover crop termination-tillage methods on corn (Zea mays L.) N credit and grain yield in a soybean (Glycine max Merr.) - winter wheat (Triticum aestivum L.)/cover crop - corn rotation over 3 consecutive years on a clay loam soil in southwestern Ontario. The cover crops were summer-seeded after wheat harvest, and included hairy vetch (Vicia villosa L. Roth), red clover (Trifolium pratense L.), white clover plus crimson clover mix (incarnatum and Trifolium alba L.), and a no cover crop control (CK). The cover crops were nested within terminationtillage method, including moldboard plow-down in fall, and herbicide spraydown in spring under strip-tillage or no-tillage then corn planting. Nitrogen fertilizer to corn was applied via side-dress at 200 kg N ha-1 to the CKs, and at 100 kg N ha-1 to the cover crops. Averaged over 3 years, above-ground biomass N level followed the pattern: hairy vetch > white clover plus crimson clover mix > red clover for fall plow-down termination. There were no significant differences in cover crop biomass N levels for spring termination. Corn response to cover crop was significantly affected by termination-tillage method and cover crop species, with poor corn stands after no-tillage, red clover and white plus crimson clover. Fall plow-down termination provided a range in N credit of 80-85 kg N ha-1, which was significantly greater than the N credit under herbicide spray-down in spring. Under spring strip-tillage and notillage, hairy vetch produced significantly greater corn grain yields than red clover and white plus crimson clover mix. [ABSTRACT FROM AUTHOR]

Soil water contents and cropping history influence soil organic matter decomposition, N cycling, and N[O.sub.3.sup.-] losses. However, soil water content fluctuates within and between seasons, which adds complexity to these biochemical processes. The impacts of constant soil water contents (30, 45, 60, 75, and 90% water-filled pore space, WFPS), wet-dry (WD) cycles (90-30% WFPS), and crop rotation (monoculture corn [Zea mays L.], a 2-yr corn-soybean [Glydne max (L.) Merr.] rotation, or a 3-yr corn--soybean--winter wheat [Triticum aestivum t.] rotation) on [N.sub.2]O emissions, inorganic N, and &nitrification enzyme activity (DEA) were investigated using repacked soil cores incubated for 50 d. The 30% WFPS treatment resulted in a net accumulation of inorganic N after 50 d. In the constant-WFPS treatments, increasing water content increased DEA, reduced soil inorganic N, and enhanced [N.sub.2]O emissions. When water contents fluctuated in five WD cycles, there was a considerable decrease in N[O.sub.3.sup.-]concentrations, high DEA activity, but low [N.sub.2]O emissions, which were similar to the emissions for the 30% WFPS treatment. Hence, fluctuating soil water content provided a soil environment that may have stimulated [N.sub.2]O reduction to [N.sub.2] or other microbial processes such as N[O.sub.3.sup.-] immobilization. At the wettest WFPS (90% WFPS), [N.sub.2]O emissions were greater under monoculture corn than the 3-yr rotation, which is consistent with the larger decreases in inorganic N and higher DEA levels. Nitrous oxide emissions accounted for between 7.5 and 27.4% of N[O.sub.3.sup.-] loss when soils were incubated at 90% WFPS but only 3.86 to 5.12% with the WD cycles. Nitrous oxide emissions were found to be influenced by soil water content to a greater extent than crop rotation. doi: 10.2136/sssaj2010.0064

Soil N mineralization is an important N contributor to crop uptake; however, the soil and climatic controls on soil mineralizabie N are poorly understood. Soil samples from 56 sites across Canada were used to determine the potential to predict the size of mineralizable N pools through simple soil properties and through simple climatic indices and the re_clim indices. Mineralizable N was determined using a 24-wk aerobic incubation at 25[degrees]C. Potentially mineralizable N ([N.sub.0]) was estimated by' curve fitting using N mineralized from 2 to 24 wk, and Pool I, a labile mineralizable N pool, was determined as the N mineralized in the first 2-wk period. Soil properties were relatively effective predictors of [N.sub.0] with soil organic N (SON) and sand explaining 40 and 34% of the variability, respectively. Particulate organic matter N (POM-N) and pH explained 18 and 25%, respectively, of the variability in Pod I. Simple climate normals were generally poor predictors of pool size except for potential evapotranspiration (PET), which predicted 24% of the variability in Pool I. There_clim indices, normally applied to the activity of soil &composers and applied here for the first time to explain soil mineralizable N pool size variability, performed better than simple climate indices and explained up to 26% of the variation in [N.sub.0]. By including soil and climatic parameters in a multiple regression model, it was possible to explain about 63 and 40% of the variability in [N.sub.0] and Pool I, respectively, across a wide range of arable soils in Canada. doi: 10.2136/sssaj2009.0213

Tillage practices may affect the active fraction of soil organic N. As part of a national project to examine soil management and environmental controls on the active fraction of organic N, this study examined the effects of no-till (NT) and conventional tillage (CT) systems on the quantity of potentially mineralizable soil N ([N.sub.0]) and mineralizable N pools, and the potential to detect changes in these pools using N availability indices. Preplant soil samples from the top 15 cm were collected from four long-term tillage experiments at Swift Current, SK; Woodslee, ON; L'Acadie, QC; and Agassiz, BC. Potentially mineralizable N was determined by aerobic incubation at 25[degrees]C and periodic leaching for 24 wk. The [N.sub.0] was greater under NT than under CT, but only at Swift Current. The labile and intermediate mineralizable N pools were significantly higher under NT than under CT at three of the four sites. The stable mineralizable N pool and the mineralization rate coefficient (k) were greater under NT than under CT at only one of the four sites. Adoption of NT influenced the quality of the active organic N fraction at three sites, as indicated by an increased proportion of mineralizable N in the more labile N pools. Among tested indices of N availability, KCl-extractable N[H.sub.4]-N, NaOH-extractable N, Illinois Soil N Test, phosphate-borate buffer extractable N, and particulate organic C were most sensitive to tillage-induced changes in the active organic N fraction. Tillage-induced changes in the size and quality of the active organic N fraction may influence soil N supply and should be considered in optimizing fertilizer N management. Abbreviations: [N.sub.0], potentially mineralizable nitrogen; NaHC[O.sub.3]-205, ultraviolet absorbance of sodium bicarbonate extract at 205 nm; NaHC[O.sub.3]-260, ultraviolet absorbance of sodium bicarbonate extract at 260 nm; CT, conventional tillage; ISNT, Illinois Soil Nitrogen Test for amino sugar nitrogen; MBC, microbial biomass carbon; NT, no-till; POM-C, particulate organic carbon; POM-N, particulate organic nitrogen; SOC, soil organic carbon; SON, soil organic nitrogen.